&NA;Changes in right ventricular systolic function directly influence left ventricular systolic function. Most of our knowledge of this systolic direct ventricular interaction comes from studies of isolated hearts, which suggest that changes in right ventricular size only affect left ventricular systolic function at low pressures and volumes. However, almost nothing is known about systolic direct ventricular interaction in a heart functioning in situ as a part of the intact circulation. We used sudden constriction of the pulmonary artery to assess the immediate effect of a change in right ventricular pressure and contraction pattern on left ventricular contraction and relaxation on the beat following the pulmonary artery constriction in anesthetized open‐chest dogs. By focusing on this first beat, we were able to avoid the confounding effect of series ventricular interaction, which changes left ventricular filling and, thus, indirectly influences left ventricular function. At baseline left ventricular end‐diastolic pressure of 9.6±2.1 mm Hg (mean±SD), sudden pulmonary artery constriction increased left ventricular peak systolic pressure by 3±2 mm Hg (2% change), left ventricular stroke volume by 2±2 ml (8% change), and monoexponential time constant of left ventricular pressure fall during relaxation by 9±6 msec (22% change). This increased left ventricular relaxation time constant was associated with altered regional segment length changes in the posterior and anterior left ventricular free walls during relaxation. We conclude that systolic direct ventricular interaction affects left ventricular systolic function and relaxation under normal conditions in the intact circulation. (Circulation Research1989;65:307‐315)